1. Field of Invention
The present invention relates to communication systems and more specifically to an architecture and tracking filters for use in a broadband tuner.
2. Prior Art
Tuners are used for both terrestrial and cable reception within TVs, VCRs, DVD recorders, set-top-boxes, FM radios, modems and other similar consumer devices. The tuner selects a narrowband signal from within a wide- or broad-band signal comprising many channels. The tuner comprises a combination of band pass filtering and frequency down conversion. One known tuner architecture employs a tunable band pass filter, also called a tracking filter, on the input signal, followed by frequency down conversion, and additional filtering at a fixed frequency. The tunable band pass filter restricts the range of frequencies that are amplified and down converted to relax the requirement of the amplifiers and to improve the rejection of unwanted channels, noise, and interference. For broadband tuner applications, the most common solution today is the can-tuner. This is typically a combination of one or two monolithic ICs and several hundred discrete components housed in a metal can.
Frequency tuning and down converting methods often use relatively high-voltage (for example 30 volts) high capacitance ratio varactors (also called varactor diodes or variable capacitance diodes) as tuning elements in filters. A disadvantage of this approach is that this conventional method is not compatible with low voltage semiconductor processes such as CMOS and results in the varactors residing off-die and a separate power supply is often required. This increases cost and space usage.
Prior art usage of on-die varactors use low voltage varactors integrated on-die and operating at the power supply rails, for example, 3 volts for CMOS. The capacitance ratio is typically 2:1 which results in a limited tuning range. Discrete off-die high voltage varactors operating at 30 volts provide for a higher capacitance ratio, for example, 10:1, and if required, can maintain a high Q over this tuning range. These varactors are known, commonly found as single or back-to-back connection of two diodes in a 3-terminal package, but do not have other passive or active components integrated together. Typically in a can-tuner, temperature and aging changes in these varactors are compensated for by the means of using (sharing) the same voltage as the one used on the varactor in the PLL. This voltage tracks the changes of the varactor in the PLL because the PLL is locked to a stable frequency reference and maintains the frequency by correcting the varactor voltage. As the varactors are discrete, they are poorly matched and so the compensation is degraded. Also because the tracking filter is operated at a different frequency to the PLL, the compensation is further degraded in the can-tuner.
Prior art use of tracking filters in can-tuners provide attenuation to large jammer signals thereby significantly reducing the linearity requirements of the front end of the tuner. By reducing the linearity requirements power consumption can be significantly reduced. This filtering also reduces the requirements of the oscillator phase noise by attenuating the unwanted signals interference effects due to reciprocal mixing. Also these can-tuners typically include a tracking notch filter to attenuate the image signal, which increases complexity and cost.
Due to component tolerances, which may have initial component errors or drift errors varying with time and temperature, the center frequency of a filter will have error. Prior art tracking filters use a filter pass band response that is flat over a frequency range wider than the desired channel bandwidth to avoid rejecting portions of the desired signal if the center frequency is in error. The wide flat response requires higher complexity filters with many components.
Prior art in this field includes the following patents and publications:
U.S. Patent Application Publication US 2003/0207672 published Nov. 6, 2003 by Dang and Egan entitled “Wideband Tuning Circuit for Low-Voltage Silicon Process and Method for Generating a Tuning Frequency”, incorporated herein by reference, describes a wideband tuning circuit suitable for low-voltage silicon process and includes a plurality of frequency band modules for generating a frequency within a particular frequency band of the tuning range.
U.S. Pat. No. 6,865,381, issued Mar. 8, 2005 to Vorenkamp, et al, entitled “System and Method for On-chip Filter Tuning”, incorporated herein by reference, describes an integrated receiver with channel selection and image rejection substantially implemented on a single CMOS integrated circuit that uses a varactor in parallel with switched capacitors to tune a VCO frequency and switched capacitors to tune filters.
U.S. Pat. No. 6,823,292, issued Nov. 23, 2004 to Spencer, entitled “Tuneable Filter”, incorporated herein by reference, describes a tunable filter that uses a single varactor in parallel with switched capacitors.
U.S. Pat. No. 5,280,638, issued Jan. 18, 1994 to Porambo, et al, entitled “RF Filter Self-alignment for Multiband Radio Receiver”, incorporated herein by reference, describes a variable frequency RF passband filter that is aligned or calibrated using reference frequencies obtained from a fixed frequency oscillator present in a different band tuning section of a multiband receiver.
U.S. Pat. No. 6,521,939, issued Feb. 18, 2003 to Yeo, et al, entitled “High Performance Integrated Varactor on Silicon”, incorporated herein by reference, describes a new MOS varactor device.
U.S. Pat. No. 6,933,789, issued Aug. 23, 2005 to Molnar, et al, entitled “On-Chip VCO Calibration”, incorporated herein by reference, provides techniques for calibrating voltage-controlled oscillators (VCOs).
U.S. Pat. No. 6,778,023, issued Aug. 17, 2004 to Christensen, entitled “Tunable Filter and Method of Tuning a Filter”, incorporated herein by reference, describes a bandpass filter that is tuned by converting the filter into an oscillator using a negative resistance circuit.
U.S. Patent Application Publication US 2005/0030108 published Feb. 10, 2005 by Duncan, et al, entitled “Integrated VCO having an Improved Tuning Range over Process and Temperature Variations”, incorporated herein by reference, describes an integrated VCO with improved tuning range.
U.S. Patent Application Publication US 2004/0184216 published Sep. 23, 2004 by Kurosawa, et al, entitled “Voltage Controlled Variable Capacitance Device”, incorporated herein by reference, describes a varactor element having an N well formed on a P type substrate.
U.S. Pat. No. 5,311,158, issued May 10, 1994 to Asbeck, et al, entitled “High Power Tuning Device Using Layered Varactors”, incorporated herein by reference, describes a matrix assembly of discrete varactors.
U.S. Pat. No. 6,307,442, issued Oct. 23, 2001 to Meyer, et al, entitled “Enhanced LC Filter with Tunable Q”, incorporated herein by reference, describes a tunable filter circuit with a feedback control circuit to tune the variable resistor in order to calibrate a quality factor of the circuit.
U.S. Pat. No. 6,885,263, issued Apr. 26, 2005 to Toncich, entitled “Tunable Ferro-Electric Filter”, incorporated herein by reference, describes an invention that quantifies and reduces losses in tunable bandpass filters having ferro-electric capacitors.
U.S. Pat. No. 6,714,776, issued Mar. 30, 2004 to Birleson, entitled “System and Method for an Image Rejecting Single Conversion Tuner with Phase Error Correction”, incorporated herein by reference, describes a tuner system that uses phase shifted in-phase and quadrature-phase signal paths as an image rejection circuit.
U.S. Pat. No. 6,731,712, issued May 4, 2004 to Lindstrom, et al, entitled “Fully Integrated Broadband Tuner”, incorporated herein by reference, describes a fully integrated single-loop frequency synthesizer.
Publication by Mujahed, entitled “High Voltage GaN Variable Capacitance Diode”, incorporated herein by reference, describes a GaN varactor diode capable of 4:1 capacitance tuning ratio in the bias range of 100 to 250 volts.
Publication by Tilmans, et al, entitled “MEMS for wireless communications: ‘from RF-MEMS components to RF-MEMS-SiP’” in the Journal of Micromechanics and Microengineering, Issue 4, July 2003, incorporated herein by reference, describes the progress in RF-MEMS.
Publication by Nath, et al, entitled “An Electronically-Tunable Microstrip Bandpass Filter Using Thin-Film Barium Strontium Titanate (BST) Varactors”, incorporated herein by reference, describes a tunable third-order combine bandpass filter using thin-film BST varactors.
U.S. Pat. No. 6,307,442 to Meyer et al., issued Oct. 23, 2001, entitled “Enhanced LC filter with tunable Q”, incorporated herein by reference, describes a tunable electronic filter circuit that tunes a variable capacitor and variable resistor to set a center frequency and quality factor.
A system-in-package (SIP) is typically made up of a substrate that carries one or more layers of patterned metal for interconnection, one or more monolithic integrated circuit (IC) die connected with either wire-bonding or flip-chip-balls and one or more discrete components, such as resistors, capacitors, inductors, diodes, transistors, and other components. The discrete components can be surface-mount devices, wire-bonded devices or components fabricated directly on the substrate.
To save space and cost, a monolithic or near-monolithic approach to a tuner with a single, readily available power supply is desirable. The need exists for a tuner that is low cost, has good selectivity, has high linearity, is adaptable to varying signal input conditions, and is insensitive to component value errors and drift.